Aqueous solutions of amine functionalized ionic compounds for carbon capture processes

ABSTRACT

An aqueous ionic absorbent solution is disclosed containing (a) about 15 wt. % to about 80 wt. % of one or more diluents, based on the total weight of the aqueous ionic absorbent solution; and (b) an ionic absorbent containing a cation and an anion comprising an amine moiety.

BACKGROUND

1. Technical Field

The present invention generally relates to aqueous ionic absorbentsolutions for carbon capture processes. The present invention furthergenerally relates to aqueous ionic absorbent solutions containing anamine functionalized ionic absorbent dissolved in a sufficient amount ofdiluent for use in a carbon dioxide capture process.

2. Description of the Related Art

The removal of carbon dioxide from the natural gas is commerciallypracticed now in order to obtain natural gas which satisfies salesspecifications or other process-dependent requirements.

Carbon dioxide is one of the primary combustion products as fuel isburned and is emitted into the atmosphere as a waste flow in the fluegas. Removal of carbon dioxide from the flue gas is not commonlypracticed at present time. As the efforts to control the CO₂ emissionsto the atmosphere increases, the removal of carbon dioxide from the fluegas may become necessary and to be practiced in an industrial scale inorder to satisfy the carbon dioxide emission requirements which are setby air pollution control authorities. The CO₂ removal process fromnatural gas may not be directly applied to the CO₂ removal from flue gassince the conditions of these two processes are very different.

Several processes for removing carbon dioxide from gases are known.Examples of such processes for carbon dioxide separation and captureinclude chemical absorption, physical and chemical adsorption,low-temperature distillation, gas-separation membranes,mineralization/biomineralization, and vegetation. The carbon dioxideabsorption process is a unit operation where one or more components in agas mixture are dissolved in a liquid (solvent). The absorption mayeither be a purely physical phenomenon or involve a chemical reaction,such as the reaction between carbon dioxide and an amine. Generally, theliquid solvent is an aqueous amine solution for the removal of carbondioxide from gas streams.

An example of an absorption process is the process for removing carbondioxide from flue gas by means of monoethanolamine (MEA) ordiethanolamine (DEA). The flue gas is led into an absorption columnwhere it comes into contact with MEA or DEA which absorbs the carbondioxide molecules. Typically, these amines, MEA and DEA, are used as 25to 30 wt. % amine in an aqueous solution. The amine solution enters thetop of an absorption tower while the carbon dioxide containing gaseousstream is introduced at the bottom. The solvent is then led to adesorption process where the liquid is heated, and the carbon dioxidemolecules are removed from the solvent by means of a desorption column.carbon dioxide and water emerge from the amine solution and the water isseparated by condensing the water vapor in a heat exchanger. The solventis cooled and then recycled back to the absorption tower for additionalcarbon dioxide absorption.

Solvent chemistry, corrosion, and viscosity consideration limit theamine strength to about 30 wt. % MEA. At flue-gas carbon dioxide partialpressures (e.g., 0.04 to 0.15 atm), the carbon dioxide-rich (“rich”)solvent loading is about 0.42 to 0.45 mol CO₂/mol MEA and the CO₂-lean(“lean”) solvent loading is about 0.15 to 0.17 mol CO₂/mol MEA. Thedifference in loading (0.25 to 0.3 mol CO₂/mol MEA) sets the circulationrate of the amine and influences capital and operating costs.

MEA also has disadvantages in that it has several mechanisms of loss,and a continuous makeup of MEA is required by post-combustion processes.For example, MEA degrades in the presence of oxygen from the flue gas.Thus, to limit the oxidative degradation, corrosion inhibitors may beused. MEA also degrades into heat-stable salts (HSS) from reaction withcarbon dioxide. To solve this problem, a reclaimer would be added on theregenerator to separate the HSS from the amine solution to providesuitable makeup MEA. Lastly, the volatility of MEA results in thetreated flue gas to contain in excess of 500 ppmv MEA when leaving theabsorber to the vent. To address this, a wash section is added at thetop of the absorber and makeup water is added to scrub the MEA from thetreated flue gas. The mixture is then sent down the column along withthe remaining lean solvent to absorb carbon dioxide from the incomingflue gas. Water washing can cut the MEA emissions to about 3 ppmv.

MEA may also degrade over time thermally, thereby limiting thetemperature of operation in the absorber and regenerator. With a cooledflue gas inlet temperature of about 56° C., the absorber column mayoperate at a bottoms temperature of 54° C. and a pressure of 1.1 barwhile the regenerator may operate at a bottoms temperature of 1.9 barand 121° C. (2 bar saturated steam). For 30 wt. % MEA, the aminereboiler steam temperature is kept at less than 150° C. (4.7 barsaturated steam) to limit thermal degradation.

MEA also degrades in the presence of high levels of NOx and SOx whichare common in facilities that burn coal and fuel oil. However, if carbondioxide removal from a high NOx and SOx containing flue gas is desired,separate process facilities such as SCR (Selective Catalytic Reduction)and FGD (Flue Gas Desulfurization) are needed for removal of NOx andSOx, respectively.

Accordingly, to have a superior, post-combustion carbon dioxide removaltechnology that is better than those known in the art (30 wt. % MEA andsimilar aqueous amines), it is desirable to develop an improved carbondioxide absorption solvent and a process for its use.

SUMMARY

In accordance with one embodiment of the present invention, there isprovided an aqueous ionic absorbent solution comprising (a) about 15 wt.% to about 80 wt. % of one or more diluents, based on the total weightof the aqueous ionic absorbent solution; and (b) an ionic absorbentcontaining a cation and an anion comprising an amine moiety.

In accordance with a second embodiment of the present invention, thereis provided an aqueous ionic absorbent solution comprising (a) about 15wt. % to about 80 wt. % of one or more diluents, based on the totalweight of the aqueous ionic absorbent solution; and (b) an ionicabsorbent containing a cation and an anion comprising an amine moiety,wherein the anion is represented by the general formula:

R¹—N(R¹)-(L)-A⁻

wherein R¹ is the same or different and includes hydrogen, a straight orbranched C₁ to C₃₀ substituted or unsubstituted alkyl group, a C₁ to C₂₀ester-containing group, a substituted or unsubstituted C₃ to C₃₀cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group,a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substitutedor unsubstituted C₅ to C₃₀ heteroaryl group, a substituted orunsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₆ to C₃₀ heteroarylalkyl group, or R and R¹ together withthe nitrogen atom to which they are bonded are joined together to form aheterocyclic group; L is a linking group; and A⁻ is an anionic moiety.

In accordance with a third embodiment of the present invention, there isprovided the use of an aqueous ionic absorbent solution comprising (a)about 15 wt. % to about 80 wt. % of one or more diluents, based on thetotal weight of the aqueous ionic absorbent solution; and (b) an ionicabsorbent containing a cation and an anion comprising an amine moiety inthe capture of carbon dioxide from a gas stream containing carbondioxide.

In accordance with a fourth embodiment of the present invention, thereis provided a process for separating carbon dioxide (CO₂) from a carbondioxide-containing gas stream, the process comprising (a) contacting thecarbon dioxide-containing gas stream with an aqueous ionic absorbentsolution under absorption conditions to absorb at least a portion of theCO₂ from the carbon dioxide-containing gas stream and form aCO₂-absorbent complex; wherein the aqueous ionic absorbent solutioncomprises (i) about 15 wt. % to about 80 wt. % of one or more diluents,based on the total weight of the aqueous ionic absorbent solution; and(ii) an ionic absorbent containing a cation and an anion comprising anamine moiety; and (b) recovering a gaseous product having a reduced CO₂content.

The present invention advantageously provides an aqueous ionic absorbentsolution containing an amine functionalized ionic absorbent dissolved ina sufficient amount of one or more diluents for use in a carbon dioxidecapture process. The aqueous ionic absorbent solution is composed of acation-anion pair, which includes a low molecular weight cation and ananion containing an amine functional group. By controlling the molecularweight and type of cation of the ionic absorbent as well as the diluentcontent, the aqueous ionic absorbent solution will retain an acceptableviscosity to facilitate rapid absorption and desorption of carbondioxide in an absorption-stripping CO₂ capture process while alsomaximizing the volumetric capacity of the aqueous ionic absorbentsolution for removing CO₂ from a process stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 shows generic synthetic schemes for preparing an ionicabsorbent in accordance with the present invention.

FIG. 5 shows a process flow diagram scheme for the removal of CO₂ from aflue gas stream in accordance with one embodiment of the presentinvention.

FIG. 6 shows the CO₂ loading results at various temperature foramine-functionalized ionic absorbents in accordance with the presentinvention.

FIG. 7 shows the CO₂ loading results at various temperature formonoethanolamine in 70 wt. % water.

DETAILED DESCRIPTION

The present invention is directed to an aqueous ionic absorbent solutioncomprising (a) about 15 wt. % to about 80 wt. % of one or more diluents,based on the total weight of the aqueous ionic absorbent solution; and(b) an ionic absorbent containing a cation and an anion comprising anamine moiety.

A suitable diluent includes, by way of example, inert diluents such aswater, monohydric alcohols, polyols, and the like and mixtures thereof.Representative examples of suitable monohydric alcohols include C₁ toC₁₂ alcohols such as methanol, ethanol, isopropanol, 1-propanol,1-butanol, 2-butanol, t-butanol, 2-methyl-1-propanol, 1-pentanol,1-hexanol, 1-heptanol, 4-heptanol, 1-octanol, 1-nonyl alcohol,1-decanol, 1-dodecanol and the like and mixtures thereof.

The polyols for use as a diluent include those having from 2 to about 10carbon atoms and from two to six hydroxyl groups. Representativeexamples of suitable polyols include glycerol, triethylene glycol,2-ethylene glycol, diethylene glycol, 1,2-propylene glycol,1,3-propylene glycol, tetraethylene glycol, hexylene glycol and the likeand mixtures thereof. In one preferred embodiment, the diluent is water.

In general, the amount of diluent added to the ionic absorbent is anamount capable of forming an absorption solvent having an acceptableviscosity to facilitate mass transfer of CO₂ into and out of the aqueousionic absorbent solution in a carbon capture process. The ionicabsorbents for use in forming the aqueous ionic absorbent solution ofthe present invention have a relatively high viscosity in the absence ofdiluent. As such, the rate of CO₂ absorption for the ionic absorbents inthe absence of one or more diluents is low and takes a relatively longtime to reach the equilibrium. It has surprisingly been discovered thatthe use of a sufficient amount of diluent, such as water, lowers theviscosity of the ionic absorbent such that the absorption rate of CO₂ issignificantly increased and equilibrium CO₂ loading is reached morequickly than without the diluents.

A suitable viscosity for the aqueous ionic absorbent solution in acarbon capture process will ordinarily be from about 0.1 to about 100centistoke (cSt). In one embodiment, a suitable viscosity for theaqueous ionic absorbent solution in a carbon capture process can rangefrom about 0.5 to about 40 cSt.

In one embodiment, the diluent content in the aqueous ionic absorbentsolution is from about 15 to about 80 wt. %, based on the total weightof the aqueous ionic absorbent solution. In one embodiment, the diluentcontent in the aqueous solution is from about 20 to about 70 wt. %,based on the total weight of the aqueous ionic absorbent solution. Inone embodiment, the diluent content in the aqueous solution is fromabout 40 to about 60 wt. %, based on the total weight of the aqueousionic absorbent solution. Surprisingly, the water content in thesolution has relatively little effect on the uptake of CO₂ measured asmoles of CO₂ per mol of ionic absorbent for the aqueous ionic absorbentsolutions of the present invention. Accordingly, the content of diluentcan be adjusted for optimum viscosity with little to no detrimentaleffects due to dilution.

It is also desirable to maintain the ionic absorbent concentrationsufficiently high in order to keep the overall absorption solutionvolume to an acceptable size for the size of absorption equipmentdesign. For example, to lower the overall cost, it is desirable to useless amount of diluent.

The ionic absorbent is generally composed of a cation and an anion. Inone embodiment, the ionic absorbent is a liquid ionic absorbent andincludes a category of compounds which are made up entirely of ions andare liquid at or below process temperatures including room temperature.The ionic liquids may have low melting points, for example, from −100°C. to 200° C. They tend to be liquid over a very wide temperature range,with a liquid range of up to about 500° C. or higher. Ionic liquids aregenerally non-volatile, with effectively no vapor pressure. Many are airand water stable, and can be good solvents for a wide variety ofinorganic, organic, and polymeric materials. In another embodiment, theionic absorbent is a solid ionic absorbent and includes a category ofcompounds which are made up entirely of ions and are solid in ananhydrous state at room temperature.

The ionic absorbent for use in forming the aqueous ionic absorbentsolution of the present invention includes a cation and an anioncomprising an amine moiety. The properties of the ionic absorbent can betailored by varying the cation and anion pairing. The amine moietyadvantageously provides selectivity for the aqueous ionic solution tocomplex with CO₂.

In order for the aqueous ionic absorbent solutions containing the ionicabsorbent to have a high volumetric/absorption capacity (mol CO₂/mLabsorbent), it is desirable to have the molecular weight of the neationic absorbent as low as possible to achieve the maximum molarconcentration of ionic absorbent in the solution per weight of ionicabsorbent basis which in turn will reduce the cost of the absorptionsolution and the equipment size for the CO₂ capturing process.

It is believed that the cation group of the ionic absorbent has littleimpact on the molar CO₂ absorption capacity. Accordingly, the molecularweight of the cation can be as low as possible thereby lowering theoverall molecular weight of the ionic absorbent. In one embodiment, themolecular weight of the cation can range from about 18 to about 500atomic mass unit (AMU) (g/mole). In another embodiment, the molecularweight of the cation can range from about 18 to about 400 atomic massunit (AMU) (g/mole).

In one embodiment, a cation is a secondary, tertiary or quaternaryphosphonium cation represented by the general formula:

wherein R is the same or different and is hydrogen, a substituted orunsubstituted alkyl group, a substituted or unsubstituted fluoroalkylgroup, a substituted or unsubstituted cycloalkyl group, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, a substituted or unsubstituted arylalkyl group, a substituted orunsubstituted heteroarylalkyl group, or —(CH₂)_(n)—R′, wherein R′represents independently for each occurrence a substituted orunsubstituted cycloalkyl group, a substituted or unsubstituted arylgroup, or a substituted or unsubstituted heteroaryl group; and nrepresents independently for each occurrence an integer in the range 1to 10 inclusive.

In one embodiment, a cation is a secondary, tertiary or quaternaryammonium cation represented by the general formula:

wherein R is the same or different and is hydrogen, a substituted orunsubstituted alkyl group, a substituted or unsubstituted fluoroalkylgroup, a substituted or unsubstituted cycloalkyl group, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, a substituted or unsubstituted arylalkyl group, a substituted orunsubstituted heteroarylalkyl group, or —(CH₂)_(n)—R′, wherein R′represents independently for each occurrence a substituted orunsubstituted cycloalkyl group, a substituted or unsubstituted arylgroup, or a substituted or unsubstituted heteroaryl group, or three Rgroups together with the nitrogen atom to which they are bonded can betaken together to represent pyridinium, imidazolium, benzimidazolium,pyrazolium, benzpyrazolium, indazolium, thiazolium, benzthiazolium,oxazolium, benzoxazolium, isoxazolium, isothiazolium, imdazolidenium,guanidinium, quinuclidinium, triazolium, tetrazolium, quinolinium,isoquinolinium, piperidinium, pyrrolidinium, morpholinium, pyridazinium,pyrazinium, piperazinium, triazinium, azepinium, or diazepinium; and nrepresents independently for each occurrence an integer in the range 1to 10 inclusive.

In another embodiment, the cation is a Group 1 or Group 2 metal of thePeriodic Table. Representative examples of Group 1 metals includelithium, sodium, potassium, rubidium, cesium and the like.Representative examples of Group 2 metals include calcium, barium,magnesium, or strontium and the like.

In one embodiment, a cation includes, but is not limited to, a Group 1or Group 2 metal of the Periodic Table, an ammonium cation, phosphoniumcation, an imidazolium cation, a pyridinium cation, a pyrazolium cation,an oxazolium cation, a pyrrolidinium cation, a piperidinium cation, analkyl thiazolium cation, an alkyl guanidinium cation, a morpholiniumcation, a trialkylsulfonium cation, a triazolium cation, and the like.

In one embodiment, a cation is a trialkyl or a tetraalkyl ammoniumcation or phosphonium cation in which the alkyl group of the trialkyl ortetraalkyl is the same or different and is a C₁ to C₃₀ straight orbranched, substituted or unsubstituted alkyl group. In anotherembodiment, a cation is a tetraalkyl ammonium cation or a tetraalkylphosphonium cation in which the alkyl group of the tetraalkyl is thesame or different and is a C₁ to C₆ straight or branched, substituted orunsubstituted alkyl group. The cation may contain ring structures wherethe N or P atom is a part of the ring structure.

Suitable anions for the ionic absorbent include those represented by thegeneral formula:

R¹—N(R¹)-(L)-A⁻

wherein R¹ is the same or different and includes hydrogen, a straight orbranched C₁ to C₃₀ substituted or unsubstituted alkyl group, a C₁ to C₂₀ester-containing group, a substituted or unsubstituted C₃ to C₃₀cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group,a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substitutedor unsubstituted C₅ to C₃₀ heteroaryl group, a substituted orunsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₆ to C₃₀ heteroarylalkyl group, or R and R¹ together withthe nitrogen atom to which they are bonded are joined together to form aheterocyclic group; L is a linking group, which can be a bond, or adivalent group selected from the group consisting of a straight orbranched C₁ to C₃₀ substituted or unsubstituted alkyl group, a C₁ to C₂₀ester-containing group, a substituted or unsubstituted C₃ to C₃₀cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group,a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substitutedor unsubstituted C₅ to C₃₀ heteroaryl group, a substituted orunsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₆ to C₃₀ heteroarylalkyl group and the like; and A⁻ is ananionic moiety.

In one embodiment, A⁻ is SO₃ ⁻ or PO₄ ⁻ or a conjugate base ofmultivalent acid.

In one embodiment, R¹ are the same or different and include hydrogen ora straight or branched C₁ to C₆ substituted or unsubstituted alkylgroup, L is a divalent straight or branched C₁ to C₆ substituted orunsubstituted alkyl group and A⁻ is SO₃ ⁻.

Representative examples of alkyl groups for use herein include, by wayof example, a straight or branched alkyl chain containing carbon andhydrogen atoms of from 1 to about 30 carbon atoms and preferably from 1to about 6 carbon atoms with or without unsaturation, to the rest of themolecule, e.g., methyl, ethyl, n-propyl, 1-methylethyl (isopropyl),n-butyl, n-pentyl, etc., and the like.

Representative examples of fluoroalkyl groups for use herein include, byway of example, a straight or branched alkyl group as defined hereinhaving one or more fluorine atoms attached to the carbon atom, e.g.,—CF₃, —CF₂CF₃, —CH₂CF₃, —CH₂CF₂H, —CF₂H and the like.

Representative examples of substituted or unsubstituted cycloalkylgroups for use herein include, by way of example, a substituted orunsubstituted non-aromatic mono or multicyclic ring system of about 3 toabout 20 carbon atoms such as, for example, cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, bridged cyclic groups or sprirobicyclic groups,e.g., spiro-(4,4)-non-2-yl and the like, optionally containing one ormore heteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted cycloalkylalkylgroups for use herein include, by way of example, a substituted orunsubstituted cyclic ring-containing group containing from about 3 toabout 20 carbon atoms directly attached to the alkyl group which arethen attached to the main structure of the monomer at any carbon fromthe alkyl group that results in the creation of a stable structure suchas, for example, cyclopropylmethyl, cyclobutylethyl, cyclopentylethyland the like, wherein the cyclic ring can optionally contain one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted cycloalkenylgroups for use herein include, by way of example, a substituted orunsubstituted cyclic ring-containing group containing from about 3 toabout 20 carbon atoms with at least one carbon-carbon double bond suchas, for example, cyclopropenyl, cyclobutenyl, cyclopentenyl and thelike, wherein the cyclic ring can optionally contain one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted aryl groups foruse herein include, by way of example, a substituted or unsubstitutedmonoaromatic or polyaromatic group containing from about 5 to about 20carbon atoms such as, for example, phenyl, naphthyl, tetrahydronapthyl,indenyl, biphenyl and the like, optionally containing one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted arylalkyl groupsfor use herein include, by way of example, a substituted orunsubstituted aryl group as defined herein directly bonded to an alkylgroup as defined herein, e.g., —CH₂C₆H₅, —C₂H₅C₆H₅ and the like, whereinthe aryl group can optionally contain one or more heteroatoms, e.g., Oand N, and the like.

Representative examples of fluoroaryl groups for use herein include, byway of example, an aryl group as defined herein having one or morefluorine atoms attached to the aryl group.

Representative examples of ester groups for use herein include, by wayof example, a carboxylic acid ester having one to 20 carbon atoms andthe like.

Representative examples of heterocyclic ring groups for use hereininclude, by way of example, a substituted or unsubstituted stable 3 toabout 30 membered ring group, containing carbon atoms and from one tofive heteroatoms, e.g., nitrogen, phosphorus, oxygen, sulfur andmixtures thereof. Suitable heterocyclic ring groups for use herein maybe a monocyclic, bicyclic or tricyclic ring system, which may includefused, bridged or spiro ring systems, and the nitrogen, phosphorus,carbon, oxygen or sulfur atoms in the heterocyclic ring group may beoptionally oxidized to various oxidation states. In addition, thenitrogen atom may be optionally quaternized; and the ring radical may bepartially or fully saturated (i.e., heteroaromatic or heteroarylaromatic). Examples of such heterocyclic ring functional groups include,but are not limited to, azetidinyl, acridinyl, benzodioxolyl,benzodioxanyl, benzofurnyl, carbazolyl, cinnolinyl, dioxolanyl,indolizinyl, naphthyridinyl, perhydroazepinyl, phenazinyl,phenothiazinyl, phenoxazinyl, phthalazinyl, pyridyl, pteridinyl,purinyl, quinazolinyl, quinoxalinyl, quinolinyl, isoquinolinyl,tetrazoyl, imidazolyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl,piperidinyl, piperazinyl, 2-oxopiperazinyl, 2-oxopiperidinyl,2-oxopyrrolidinyl, 2-oxoazepinyl, azepinyl, pyrrolyl, 4-piperidonyl,pyrrolidinyl, pyrazinyl, pyrimidinyl, pyridazinyl, oxazolyl, oxazolinyl,oxazolidinyl, triazolyl, indanyl, isoxazolyl, iso-oxazolidinyl,morpholinyl, thiazolyl, thiazolinyl, thiazolidinyl, isothiazolyl,quinuclidinyl, isothiazolidinyl, indolyl, isoindolyl, indolinyl,isoindolinyl, octahydroindolyl, octahydroisoindolyl, quinolyl,isoquinolyl, decahydroisoquinolyl, benzimidazolyl, thiadiazolyl,benzopyranyl, benzothiazolyl, benzooxazolyl, furyl, tetrahydrofurtyl,tetrahydropyranyl, thienyl, benzothienyl, thiamorpholinyl,thiamorpholinyl sulfoxide, thiamorpholinyl sulfone, dioxaphospholanyl,oxadiazolyl, chromanyl, isochromanyl and the like and mixtures thereof.

Representative examples of heterocycloalkyl groups for use hereininclude, by way of example, a substituted or unsubstituted heterocylicring group as defined herein directly bonded to an alkyl group asdefined herein. The heterocycloalkyl moiety may be attached to the mainstructure at carbon atom in the alkyl group that results in the creationof a stable structure.

Representative examples of heteroaryl groups for use herein include, byway of example, a substituted or unsubstituted heterocyclic ring groupas defined herein. The heteroaryl ring radical may be attached to themain structure at any heteroatom or carbon atom that results in thecreation of a stable structure.

Representative examples of heteroarylalkyl groups for use hereininclude, by way of example, a substituted or unsubstituted heteroarylring group as defined herein directly bonded to an alkyl group asdefined herein. The heteroarylalkyl moiety may be attached to the mainstructure at any carbon atom from the alkyl group that results in thecreation of a stable structure.

It will be understood that the term “substituted with” includes theimplicit proviso that such substitution is in accordance with permittedvalence of the substituted atom and the substituent, and that thesubstitution results in a stable compound, e.g., which does notspontaneously undergo transformation such as by rearrangement,cyclization, elimination, or other reaction. Representative examples ofsuch substituents include, but are not limited to, hydrogen, fluorine,hydroxyl groups, halogen group, carboxyl groups, cyano groups, nitrogroups, oxo (═O), thio(═S), substituted or unsubstituted alkyl,substituted or unsubstituted fluoroalkyl, substituted or unsubstitutedalkoxy, substituted or unsubstituted alkenyl, substituted orunsubstituted alkynyl, substituted or unsubstituted aryl, substituted orunsubstituted arylalkyl, substituted or unsubstituted cycloalkyl,substituted or unsubstituted cycloalkenyl, substituted or unsubstitutedamino, substituted or unsubstituted aryl, substituted or unsubstitutedheteroaryl, substituted heterocycloalkyl ring, substituted orunsubstituted heteroarylalkyl, substituted or unsubstituted heterocyclicring, and the like.

Representative examples of ionic absorbents for use in the aqueous ionicabsorbent solutions of the present invention includes tetrabutylammoniumN-propyl-N-(3-sulfopropyl)amine, tetrabutylphosphoniumN-isopropyl-N-(3-sulfopropyl)amine, tetraethylammoniumN-isopropyl-N-(3-sulfopropyl)amine and the like and mixtures thereof.

The ionic absorbents for use in the process of the present invention areknown and can be prepared by methods known in the art, e.g., asdisclosed in U.S. Pat. Nos. 7,208,605 and 7,744,838 and WO 2008/122030,the contents of which are incorporated by reference herein. For example,in one embodiment, the ionic absorbents can be prepared in accordancewith the synthetic schemes generally represented in FIGS. 1 and 2(reactions with primary amines, secondary amines or diamines) via azwiterionic intermediate. In one embodiment, the ionic absorbents can beprepared in accordance with the synthetic scheme generally representedin FIG. 3 in which the zwiterionic intermediate can be reacted with anepoxide. In one embodiment, the ionic absorbents can be prepared inaccordance with the synthetic scheme generally represented in FIG. 4.

While the above examples have shown the reaction of the zwiterionicintermediates with ammonium hydroxide salts, as the base, one may alsouse other cations as described above, e.g., a phosphonium cation, aheterocyclic (e.g., imidazolium or pyridinium) cation, alkali metalcation or an alkaline earth metal cation as the counterion. In certainembodiments, the cations can be metal cations, such as Na, K, Ca, Ba,etc.

While the selected synthentic routes described above have all suggestedreacting hydroxide salts of various cations with the zwitterions, othersynthetic approaches can be envisioned as well, such as zwitteriondeprotonation with strong bases like NaH or BuLi, followed by an ionmetathesis step to exchange the Na or Li for a different cation.

As discussed above, in order for the aqueous ionic absorbent solutionsto have a sufficiently high volumetric adsorption capacity for CO₂ (molCO₂/mL absorbent), the molecular weight of the neat ionic absorbentshould be as low as possible, e.g., a molecular weight of no more thanabout 700 atomic mass unit (AMU) (g/mole). In one embodiment, themolecular weight of the neat ionic absorbent is from about 75 to about700 AMU (g/mole). In one embodiment, the molecular weight of the neationic absorbent is below about 600 AMU (g/mole). In another embodiment,the molecular weight of the neat ionic absorbent is from about 75 toabout 600 AMU (g/mole). In one embodiment, the molecular weight of theneat ionic absorbent is below about 500 AMU (g/mole). In anotherembodiment, the molecular weight of the neat ionic absorbent is fromabout 75 to about 500 AMU (g/mole).

The aqueous ionic absorption solutions are particularly useful in acarbon capture process. In general, water vapor is present in carbondioxide-containing gas streams such as a flue gas stream at varyingconcentrations from the combustion processes. Due to its presence in theflue gas and its low cost, water is the most desirable diluent for theionic absorbent. In one embodiment, the use of the aqueous ionicabsorbent solutions of the present invention involves (a) contacting agas stream containing carbon dioxide with the aqueous ionic absorbentsolution under absorption conditions to absorb at least a portion of thecarbon dioxide from the gas stream and form a carbon dioxide-absorbentcomplex; and (b) recovering a gaseous product having a reduced CO₂content.

Suitable incoming gas streams typically contain between about 0.03 toabout 80% by volume carbon dioxide. In one embodiment, suitable incominggas streams contain between about 1 to about 33% by volume carbondioxide. In one embodiment, suitable incoming gas streams containbetween about 3 to about 15% by volume carbon dioxide.

The types of incoming gas streams that can be treated include, but arenot limited to, flue gas streams from power plants such as coal-firedpower plants, natural gas combined cycles, natural gas boilers, naturalgas, gas streams from gasification plants, gas from cementmanufacturing, reformate gas, synthesis gas, refinery off-gas, biogasand air (e.g., in a space application). If required, the incoming gasstream can be pretreated prior to entering the apparatus (e.g.,fractionation, filtration, scrubbing to remove particulates and othergaseous components, and combination or dilution with other gases).Accordingly, the chemical composition may also vary considerably.

The incoming gas streams may further contain other gases such as, forexample, O₂, NO_(x) and SO_(x). Therefore, the use of the aqueous ionicabsorbent solutions according to the present invention may alsoco-absorb one or more of SO₂, COS, NO_(X), COS, and SO_(x) present inthe gas stream. Thus, the aqueous ionic absorbent solutions may beemployed to capture all or some of the pollutants in addition to CO₂which are present in the gas stream.

In one embodiment, the use of the aqueous ionic absorbent solution ofthe present invention will be explained in more detail on the basis ofthe accompanying figure and the following examples.

FIG. 5 illustrates a process scheme according to one embodiment of thepresent invention. In general, FIG. 5 includes a carbon dioxide (CO₂)separation system for recovering high purity CO₂ from flue gas stream(A). First, flue gas stream (A) enters blower (B), which blows flue gasstream (A) to absorber (D). Blower (B) can be any type of blower knownto one skilled in the art. Generally, the pressure of flue gas stream(A) is around 1 bar. Blower (B) raises the pressure of the flue gasstream to a pressure ranging from 1.1 to about 1.5 bar. The pressure istypically raised to overcome the pressure drop associated with flowingthe gas through the absorber tower.

In one embodiment, absorber (D) is a packed tower. The packing may berandom packing, structured packing or any combination thereof. Therandomly packed material can include, but is not limited to, Raschigrings, saddle rings, Pall rings, or any other known type of packingring, or combination of packing rings. The structured packed materialcan include, but is not limited to, corrugated sheets, crimped sheets,gauzes, grids, wire mesh, monolith honeycomb structures, or anycombination thereof. Examples of structured packing include Sulzer DX™,Mellapak™, Mellapak Plus™, Katapak™, and the like.

As discussed above, water present in the flue gas stream will form anaqueous solution containing the ionic absorbent. If the viscosity of theaqueous solution is not sufficient for use in the process of the presentinvention, then it may be necessary to add one or more diluents to theaqueous solution to further reduce its viscosity to a viscosity suitablefor the process of the present invention. Accordingly, a diluent stream(G) is present at the top of the column to further hydrate the incomingionic liquid stream (C) and reduce its viscosity as discussed above. Inaddition, diluent stream (G) can scrub any additional diluent that maybe carried over into treated flue gas stream (A1) leaving the absorber(D) to partly or completely remove any unwanted impurities. Typically,the total amount of water and diluents, when present, in the ionicabsorbent is largely dictated by viscosity requirements, and isgenerally less than about 80 wt. %. In one embodiment, the total amountof water present is from about 15 wt. % to about 80 wt. %.

The flue gas stream (A) and ionic absorbent stream (C) are contacted inabsorber (D). In general, flue gas stream (A) is introduced intochemical absorber (D) and during the process of flow from the bottom up,the carbon dioxide in flue gas stream (A) is absorbed by ionic absorbent(C) flowing from the top down. The end gas, i.e., treated flue gasstream (A1), which is essentially depleted of carbon dioxide, isintroduced out of the absorber, for example, through a vent from the topof the absorber after contacting (or being scrubbed) by diluent streamG. In one embodiment, from about 80 to about 95% of CO₂ in stream A hasbeen removed to form stream A1.

The ionic absorbent stream (E), which has absorbed carbon dioxide(CO₂-rich aqueous ionic absorbent solution stream), comes out ofabsorber (D) from the bottom and is pumped to cross exchanger (K), whereits temperature is raised; and then the preheated stream (F) is pumpedthrough pump (W2). Pump (W2) raises the pressure of ionic absorbentstream (E) to greater than about 1.9 bar to provide pressurized ionicabsorbent stream (F1). In one embodiment, the pressure of ionicabsorbent stream (E) is raised to a pressure of greater than about 1.9bar to about 10 bar. In one embodiment, the pressure is raised to about4 bar to about 8 bar. Pump (W2) can be any apparatus capable of raisingthe pressure of preheated stream (F) such as, for example, a centrifugalpump, a reciprocating pump, a gear pump, etc.

Next, pressurized ionic absorbent stream (F1) is sent to the top ofregenerator (H) to flow down through the regenerator where it is furtherheated to a temperature ranging from about 90° C. to about 200° C. by,for example, steam-heated or hot-oil heated reboiler and under vacuum,atmospheric or high pressure conditions. In one embodiment, pressurizedionic absorbent stream (F1) is sent to the top of regenerator (H) toflow down through the regenerator where it is further heated to atemperature of about 120° C. to about 180° C. In this manner, most ofthe carbon dioxide in the ionic liquid (F1) is released as a wet carbondioxide gas stream (L) and emitted out from the top of regenerator (H).Regenerator (H) is a packed tower and can be any random or structurepacking as discussed above with absorber (D). The high temperatureregeneration allows for greater flexibility in using different levels ofsteam and/or waste heat resources in reboiler (Q), as discussed below.The regenerator (H) is typically run at relatively high pressure, e.g.,a pressure from about 1.9 bar to about 10 bar. By running regenerator(H) at a relatively high pressure, carbon dioxide stream (L) may berecovered at higher pressure thereby reducing the capital and operatingcosts associated with carbon dioxide compression (Y), discussed below.The ionic absorbent depleted of carbon dioxide, i.e., ionic absorbentstream (S), is emitted out from the bottom of regenerator (H).

In one embodiment, when flue gas stream (A) is first pre-cooled to atemperature of about 40° C. to about 60° C. and then sent to theabsorber and contacted with ionic absorbent stream (C), the desorptionconditions for removing carbon dioxide from pressurized ionic absorbentstream (F1) may include heating stream (F1) to a temperature of greaterthan 60° C. to about 80° C. in the case where the regenerator (i.e.,stripper) is run at vacuum conditions. In another embodiment, when fluegas stream (A) is first pre-cooled to a temperature of about 40° C. toabout 60° C. and then sent to the absorber and contacted with ionicabsorbent stream (C), the desorption conditions for removing carbondioxide from pressurized ionic absorbent stream (F1) may include heatingstream (F1) to a temperature of greater than 100° C. to about 120° C. inthe case where the regenerator is run at atmospheric conditions. In yetanother embodiment, when flue gas stream (A) is first pre-cooled to atemperature of about 40° C. to about 60° C. and then sent to theabsorber and contacted with ionic absorbent stream (C), the desorptionconditions for removing carbon dioxide from pressurized ionic absorbentstream (F1) may include heating stream (F1) to a temperature of about120° C. to about 200° C. in the case where the regenerator is run athigh-pressure conditions (i.e., 1.9 to 10 bar).

In one embodiment, when flue gas stream (A) is sent to the absorber at atemperature of about 60° C. to about 80° C., the desorption conditionsfor removing carbon dioxide from pressurized ionic absorbent stream (F1)may include heating stream (F1) to a temperature of about 100° C. toabout 200° C. in the case where the regenerator is run at atmospheric orhigh-pressure conditions.

In one embodiment, when flue gas stream (A) is sent to the absorber at atemperature of about 80° C. to about 100° C., the desorption conditionsfor removing carbon dioxide from pressurized ionic absorbent stream (F1)may include heating stream (F1) to a temperature of about 120° C. toabout 200° C. in the case where the regenerator is run at atmospheric orhigh-pressure conditions.

The wet carbon dioxide gas stream (L) coming from the top of regenerator(H) is then passed through cooler (M), such as a condenser, where it iscooled to provide cooled carbon dioxide gas stream (N). The temperatureof carbon dioxide gas stream (L) is generally decreased to about 30° C.to about 50° C. The cooled carbon dioxide gas stream (N) is sent toseparator (O) where condensed water and trace amounts of ionic absorbentand optional diluents is separated and returned to regenerator (H) asreflux (P1). The gas stream (R), as the carbon dioxide product, is sentto gas injection regulatory system (X) where it is dehydrated usingmethods known in the art, e.g., triethylene glycol dehydration orheatless absorption using molecular sieves. The dehydrated gas is thensent to compressor (Y) to be pressurized, under normal pressure and atemperature of less than about 60° C., to about 7.4 MPa or higher, andsent off to pipeline (Z). In one embodiment, the captured CO₂ can beused on-site or can be made available for sale to a co-located facility.Dried CO₂ will be compressed in a series of compressors and intercoolersto a final temperature of about 40° C. to about 60° C. The lastcompression stage would be close to the supercritical pressure of CO₂(i.e., about 1100 psig). Once CO₂ is supercritical, it may be pumped asa dense phase fluid to any pressure required for transportation—finaldense phase pressure may range from about 100 to about 200 bar.

The reboiler (Q) is a shell and tube heat exchanger. Ionic absorbentstream (S) coming from the bottom of the regenerator (H) enters into thetubes of the reboiler (Q) where it is heated by steam in the shell-sideof the reboiler. Stream (T) is the supply heating medium, such as steamthat it is available from the facility generating the flue gas (e.g.,refinery, gas/oil-fired boiler, power plant, etc.) while stream (U) isthe return condensate that is returned back to the utility system of thefacility. Therefore, the ionic absorbent stream (S) is heated in thereboiler (Q) and at least a portion of the carbon dioxide and watervapor present therein is released out and leaves from the top ofreboiler (Q) into regenerator (H) as stripping gas (I). On the otherhand, ionic absorbent solution (J) with significantly decreased contentof absorbed carbon dioxide (also referred to as “CO₂-lean absorbentsolution”) is sent back to cross exchanger (K).

The following non-limiting examples are illustrative of the presentinvention.

Experiments were conducted to measure the CO₂ absorption of aqueousionic solutions according to the present invention and to demonstratetheir effectiveness.

The equilibrium CO₂ carrying capacity of the aqueous ionic absorbentsolutions was measured via a volumetric method. A known quantity ofionic absorbent-water mixture is injected into a sealed pressure vesselcontaining high purity CO₂ gas at pressure of approximately 15 psia, andthe vessel is shaken to provide mixing of the solution. The pressure inthe vessel decreases as CO₂ is absorbed into the solution, and thepressure is monitored until the system reaches an equilibrium pressure.The temperature of the system is controlled and monitored, and anequilibrium pressure is measured for multiple temperatures in eachexperiment. The initial and final pressure of CO₂, the volume of thepressure vessel, and the quantity and composition of the injected ionicabsorbent-water mixture are all known.

The measured experimental data are used to calculate the loading of CO₂in the solution, which is reported as moles of CO₂ bound per mol ofionic absorbent in the solution. An aqueous ionic absorbent solution iscontacted in a sealed vessel with a low partial pressure CO₂ gassimulating a flue gas stream. The vessel is maintained at a constanttemperature by way of heating tape and is shaken to allow good contactbetween CO₂ and the absorbent solution until the system reachesequilibrium. When the pressure reaches the steady state, then a pressureis recorded and the vessel temperature is changed to the next set point.Based on the equilibrium CO₂ pressure, the loading per mole of ionicabsorbent is calculated. The measured pressure in the vessel is used todetermine the moles of gaseous CO₂ present in the vessel before andafter CO₂ absorption occurs (before and after the solvent is injectedinto the vessel) using the ideal gas equation of state: P_(CO2) V=n R T,where P_(CO2)=partial pressure of CO₂ (psia), V=volume of the gas phasein the vessel, n=total moles of CO₂ gas, T=measured experimentaltemperature in degrees Kelvin, and R is the Ideal Gas Constant, whichhas units of energy per mol per degree Kelvin (approximately 8.314 J/molK).

The water vapor pressure in the vessel is determined assuming thesolvent is an ideal mixture of water and absorbent using the known vaporpressure of water at the temperature of a given experiment (taken fromthe NIST steam tables, see http://webbook.nist.gov/chemistry/fluid): thepartial pressure of water in the vessel is calculated as the vaporpressure of water multiplied by the mol fraction of water in the solvent(which is known from the mass of water and absorbent used in preparingthe solvent mixture). Because the ionic materials used as absorbentshave negligibly low vapor pressure, the gas in the vessel is composedonly of water and CO₂. The partial pressure of CO₂ is calculated as thedifference between the measured total pressure and the water vaporpressure. The difference between the gas-phase quantity of CO₂ beforeand after solvent injection is used to calculate the absorbed quantityof CO₂ for various temperatures in a single experiment. The absorbedquantity of CO₂ is then divided by the quantity of ionic material in thesolvent to calculate the loading as moles of CO₂ absorbed per mole ofionic absorbent.

Example 1

The effect of water addition to the ionic absorbents was studied usingtetraethylammonium N-isopropyl-N-(3-sulfopropyl)amine (TEA) as the ionicabsorbent. Neat TEA is a very viscous material. The addition of waterlowered the viscous nature of the TEA significantly. Varying amounts ofwater was added to the TEA and the time to reach the equilibrium CO₂uptake was determined. The results are summarized below in Table 1.

TABLE 1 Water Diluent, Time to reach equi- Viscosity (cSt) Viscosity(cSt) Wt. % librium CO₂ uptake at 20° C. at 80° C. 0 >>2 days Veryviscous — 25 ~24 hours Viscous — 50 <1 hour 10.5 1.7As the data show, a neat ionic absorbent (i.e., TEA) provided asignificantly longer time to approach equilibrium as compared to (1) theaqueous ionic absorbent solution containing TEA with 25 wt. % water and(2) the aqueous ionic absorbent solution containing TEA with 50 wt. %water, i.e., 2 days versus 24 hours and 1 hour, respectively. The lowerviscosity of the aqueous ionic absorbent solution case facilitates rapidmass transfer of CO₂ between the gas and liquid phases, and enables CO₂absorption to occur on a faster timescale as compared to the neat ionicabsorbent which is mass-transfer limited due to the high viscosity ofthe absorbent. These results demonstrate that sufficient diluent contentis necessary to achieve CO₂ removal from gas streams on anindustrially-relevant timescale.

Example 2

Two 50 wt. % aqueous ionic absorbent solutions of ionic absorbent wereprepared. The ionic absorbents were tetraethylammoniumN-isopropyl-N-(3-sulfopropyl)amine (TEA) and tetramethylammoniumN-isopropyl-N-(3-sulfopropyl)amine (TMA), respectively. The aqueousionic absorbent solution was contacted in a sealed vessel with a lowpartial pressure CO₂ gas simulating a model flue gas stream. The vesselwas maintained at a constant temperature and is shaken to allow goodcontact between CO₂ and the absorbent ionic absorbent solution until thesystem reached equilibrium. When the pressure reached the steady state,then a pressure was recorded and then the vessel temperature was changedto the next set point. Based on the equilibrium CO₂ pressure, theloading per mole of ionic absorbent was calculated. The CO₂ loading ofthe two ionic absorbents were plotted as a function of CO₂ partialpressure in FIG. 6. The data in this figure demonstrate that thesesolutions have high molar absorption capacities ranging from 0.5 toabout 0.85 CO₂ absorption loading per mole of ionic absorbent. Theseabsorption capacities are significantly higher than the conventionalamine-based solvents such as monoethanolamine (MEA). The CO₂ loading for30 wt. % MEA was also measured and the data are presented in FIG. 7which shows 0.4 to 0.5 mol CO₂ absorbed per mole of MEA (comparativeexample).

Example 3

From the data in FIG. 6, the expected CO₂ removal capacity of the ionicabsorbent from flue gas can be estimated. The plot shows that an aqueousionic absorbent solution of TMA with 50 wt. % water can absorb up to0.74 mol CO₂ per mol ionic absorbent at 40° C. with a CO₂ partialpressure of 2.8 psia. This condition is reflective of the “rich”solution loading in the absorber. The solution absorption capacity forCO₂ at 95° C. and a CO₂ partial pressure of 5.0 psia decreases to 0.5mol CO₂ per mol aqueous ionic solution. This second condition mayreflect the “lean” solution loading achieved in the regeneration column.Therefore, the absorber-regenerator system described herein could remove0.24 moles of CO₂ per mol of TMA ionic absorbent circulated through theabsorber-regenerator system per pass.

Since the ionic absorbent for use in the aqueous ionic absorbentsolution can be operated in a wide range of temperatures, the CO₂capture process conditions can be chosen to increase the loadingdifference between the “rich” and “lean” CO₂ capacities. As one skilledin the art will readily appreciate, the chemical absorption capacity ofsolvent decreases with (1) decreasing partial pressure of the absorbedspecies in the gas phase and (2) increasing temperature. Therefore, byoperating the stripper at temperatures in excess of 95° C., the CO₂removal capacity of the system can be further increased. The removalcapacity of the system can also be increased by operating the stripperwith lower CO₂ partial pressure.

Therefore, our results show that that TMA would have the capacity toremove at least 0.24 moles of CO₂ per mol of ionic absorbent circulatedthrough the absorber-regenerator system per pass.

Example 4

From the data in FIG. 6, the expected CO₂ removal capacity of TEA fromflue gas can be estimated. The plot shows that an aqueous ionicabsorbent solution of TEA with 50 wt. % water absorb up to 0.76 mol CO₂per mol ionic absorbent at 40° C. with a CO₂ partial pressure of 1.2psia. This condition is reflective of the “rich” solution loading in theabsorber. In this TEA-based solution, the absorption capacity for CO₂ at95° C. and a CO₂ partial pressure of 6.3 psia decreases to 0.57 mol CO₂per mol aqueous ionic solution. This second condition may reflect the“lean” solution loading achieved in the regeneration column. Therefore,the absorber-regenerator system described herein could remove 0.19 molesof CO₂ per mol of aqueous ionic absorbent solution circulated throughthe absorber-regenerator system. As in the previous example, the CO₂removal capacity of the system will be further increased by operatingthe stripper at higher temperatures and/or operating the stripper atlower CO₂ partial pressure.

Therefore, our results show that TEA would have the capacity to removeat least 0.19 moles of CO₂ per mol of ionic absorbent circulated throughthe absorber-regenerator system per pass.

Comparative Example A

FIG. 7 shows measured CO₂ absorption data for an aqueous solutioncontaining 30 wt. % MEA with 70 wt. % water. At 20° C., the absorptioncapacity for CO₂ at a CO₂ partial pressure of 1.5 psia was approximately0.5 mol CO₂ per mol MEA, which reflects the loading in the absorptioncolumn. By heating the solution at 95° C. and a CO₂ partial pressure of3.0 psia, the measured loading decreases to 0.4 mol CO₂ per mol of MEA.This second condition reflects the “lean” solution loading achieved inthe regeneration column. Therefore, the absorber-regenerator systemusing an aqueous solution of MEA under these conditions could remove0.10 moles of CO₂ per mol of MEA. When comparing Comparative Example Awith Examples 2 and 3, the capacities of aqueous ionic absorbentsolution containing TEA and TMA, respectively, for CO₂ are higher thanthat of aqueous MEA, measured as moles of CO₂ captured per mol ofabsorbent.

It will be understood that various modifications may be made to theembodiments disclosed herein. Therefore the above description should notbe construed as limiting, but merely as exemplifications of preferredembodiments. For example, the functions described above and implementedas the best mode for operating the present invention are forillustration purposes only. Other arrangements and methods may beimplemented by those skilled in the art without departing from the scopeand spirit of this invention. Moreover, those skilled in the art willenvision other modifications within the scope and spirit of the claimsappended hereto.

1. An aqueous ionic absorbent solution comprising (a) about 15 wt. % toabout 80 wt. % of one or more diluents, based on the total weight of theaqueous ionic absorbent solution; and (b) an ionic absorbent containinga cation and an anion comprising an amine moiety.
 2. The aqueous ionicabsorbent solution of claim 1, wherein the one or more diluents comprisewater.
 3. The aqueous ionic absorbent solution of claim 1, wherein theone or more diluents comprise a diluents selected from the groupconsisting of a monohydric alcohol, a polyol and mixtures thereof. 4.The aqueous ionic absorbent solution of claim 1, wherein the amount ofthe one or more diluents is from about 25 wt. % to about 70 wt. %diluent, based on the total weight of the aqueous ionic absorbentsolution.
 5. The aqueous ionic absorbent solution of claim 1, whereinthe cation comprises one or more cations selected from the groupconsisting of an ammonium cation, a phosphonium cation, a Group 1 metalcation and a Group 2 metal cation.
 6. The aqueous ionic absorbentsolution of claim 1, wherein the cation comprises one or more cationsselected from the group consisting of a quaternary ammonium cation, aquaternary phosphonium cation, a Group 1 metal cation and a Group 2metal cation.
 7. The aqueous ionic absorbent solution of claim 1,wherein the cation has a molecular weight of about 18 to about 500atomic mass unit (AMU).
 8. The aqueous ionic absorbent solution of claim1, wherein the anion comprising an amine moiety is represented by thegeneral formula:R¹—N(R¹)-(L)-A⁻ wherein R¹ is the same or different and includeshydrogen, a straight or branched C₁ to C₃₀ substituted or unsubstitutedalkyl group, a C₁ to C₂₀ ester-containing group, a substituted orunsubstituted C₃ to C₃₀ cycloalkyl group, a substituted or unsubstitutedC₃ to C₃₀ cycloalkylalkyl group, a substituted or unsubstituted C₃ toC₃₀ cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ arylgroup, a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, asubstituted or unsubstituted C₅ to C₃₀ heteroaryl group, a substitutedor unsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₆ to C₃₀ heteroarylalkyl group, or R and R¹ together withthe nitrogen atom to which they are bonded are joined together to form aheterocyclic group; L is a linking group; and A⁻ is an anionic moiety 9.The aqueous ionic absorbent solution of claim 8, wherein R¹ is the sameor different and is a straight or branched C₁ to C₆ substituted orunsubstituted alkyl group, and L is a divalent straight or branched C₁to C₆ substituted or unsubstituted alkyl group.
 10. The aqueous ionicabsorbent solution of claim 1, wherein the ionic absorbent is selectedfrom the group consisting of tetrabutylammoniumN-propyl-N-(3-sulfopropyl)amine, tetrabutylphosphoniumN-isopropyl-N-(3-sulfopropyl)amine, tetraethylammoniumN-isopropyl-N-(3-sulfopropyl)amine and mixtures thereof.
 11. The aqueousionic absorbent solution of claim 1, wherein the molecular weight of theionic absorbent is from about 75 to about 700 AMU.
 12. The aqueous ionicabsorbent solution of claim 1, wherein the molecular weight of the ionicabsorbent is from about 75 to about 600 AMU.
 13. The aqueous ionicabsorbent solution of claim 1, wherein the molecular weight of the ionicabsorbent is from about 75 to about 500 AMU.
 14. The aqueous ionicabsorbent solution of claim 1, wherein the molecular weight of the ionicabsorbent is from about 75 to about 400 AMU.
 15. The aqueous ionicabsorbent solution of claim 1, wherein the molecular weight of the ionicabsorbent is from about 75 to about 300 AMU.
 16. The aqueous ionicabsorbent solution of claim 1, having a viscosity of about 0.1 to about100 centistokes (cSt).
 17. The aqueous ionic absorbent solution of claim1, having a viscosity of about 0.5 to about 40 cSt.
 18. The aqueousionic absorbent solution of claim 3, having a viscosity of about 0.1 toabout 100 cSt.
 19. A process for separating carbon dioxide (CO₂) from acarbon dioxide-containing gas stream, the process comprising (a)contacting the carbon dioxide-containing gas stream with an aqueousionic absorbent solution under absorption conditions to absorb at leasta portion of the CO₂ from the carbon dioxide-containing gas stream andform a CO₂-absorbent complex; wherein the aqueous ionic absorbentsolution comprises (i) about 15 wt. % to about 80 wt. % of one or morediluents, based on the total weight of the aqueous ionic absorbentsolution; and (ii) an ionic absorbent containing a cation and an anioncomprising an amine moiety; and (b) recovering a gaseous product havinga reduced CO₂.
 20. The process of claim 19, wherein the carbondioxide-containing gas stream is flue gas and the aqueous ionicabsorbent solution absorbs higher than about 0.5 mol CO₂ per mol ionicabsorbent at 40° C. with a CO₂ partial pressure in the range of 1 to 20psia.